Shauna Burton was just six weeks out of custody when detectives allege she inflicted fatal injuries on an elderly man and severely injured his ailing wife in what authorities described as a brutal crime scene inside the couple’s Rancho Cordova mobile home Sunday.

Less than a day later, Burton, 36, again was locked up, accused of murder in connection with the violent death of 78-year-old Mel Bain. His wife, Jean, remained in critical condition Monday as the result of injuries suffered in the morning attack, and her prognosis was uncertain, detectives said.

What prompted the violence remains unknown. Sacramento County sheriff’s Sgt. Lisa Bowman said the suspect was an “acquaintance” of the Bains, and that Mel Bain occasionally gave the woman money. The nature of their relationship, however, is still being investigated, Bowman said.

Burton was seen often enough around the Briarwood Mobile Home Park, where the Bains had lived for three decades, that neighbors knew her name, Bowman said. Talk about the woman and a recent mysterious visit by her and an unidentified man surfaced quickly Sunday after a neighbor came upon the crime scene.

That neighbor, who saw the front door ajar and went to check on the couple, called Rancho Cordova police about 11:40 a.m. Mel Bain was pronounced dead at the scene, having suffered blunt force trauma, Bowman said. Bain’s wife, also 78 and in deteriorating condition as the result of dementia, also had suffered trauma and was rushed to the hospital.

Early Monday morning, detectives arrested Burton on suspicion of murder, assault with a deadly weapon, robbery, car theft and violation of probation. By then, they had pieced together a series of crimes they attributed to Burton, beginning early Sunday and ending in the fatal assault.

Sometime before 7 a.m., Burton stole a maroon Cadillac Eldorado from a neighbor on Aries Way and drove to a CVS Pharmacy on Zinfandel Drive, where she claimed to have a gun and demanded cash, Bowman said. The robbery was reported to police at 7:12 a.m.

Witnesses gave police descriptions of the suspect and the car she fled in, including a license plate. The car’s owner hadn’t even realized it was stolen until police called to ask about it.

The Cadillac was found abandoned about 9:30 p.m. Sunday. However, detectives believe Burton used it to visit the Bains’ home sometime between the robbery and the discovery of the crime scene , Bowman said. The mobile home park is about three miles from the pharmacy and less than two from Burton’s home.

Nobody answered the door at Burton’s home Monday afternoon. At the Main Jail, Burton was declining media interviews. Efforts by The Bee to reach her relatives were not successful. She is scheduled to be arraigned today.

Burton has a lengthy criminal history in Sacramento County, dating back to 1997, according to Superior Court records available online. But the current allegations, and the criminal case that put her behind bars this summer, mark a significant escalation: Burton’s first four convictions were for misdemeanor offenses, three of them petty theft.

Then, in July, prosecutors charged Burton with felony domestic violence and assault with a deadly weapon. In court documents, they alleged she assaulted an unidentified cohabitant with a box cutter.

The assault charge was dropped, but Burton pleaded no contest to the domestic violence. On July 30, she was sentenced to 120 days of jail and five years of formal, searchable probation, according to court documents.

Burton served time at the Rio Cosumnes Correctional Center before being released Sept. 20, Bowman said.

She might have been accompanied by a man during the morning’s robbery, but as of Monday she was the lone suspect in the assault in Briarwood, Bowman said.

There, word of an arrest Monday offered some relief to residents shaken by the bloodshed in their otherwise friendly, close-knit community.

Still, Roseann Greco, who has lived there 12 years, said residents worry about the future of Jean Bain, who had been reliant on her husband and kindhearted neighbors since the onset of her dementia.

The couple’s daughter died in 2011, their nephew, Gary O’Connor, told The Bee on Sunday. Their remaining family include O’Connor, his cousins and a brother of Jean Bain.

“Everybody wants her to get well,” said Grego, 52. “Everybody’s concerned. Hopefully she’ll survive it and if she does, where is she going to go?”